The use of nanocrystals, also referred to as nanoparticles, in both research and commercial applications is growing dramatically. Their diverse optical, chemical and electrical properties make them attractive for use in many applications such as in semiconductor devices, photodetectors, lasers, medical imaging and pathogen detection systems and materials processing methodologies and associated systems. The various applications in which nanocrystals may be used depend in no small measure upon the nanocrystal composition, associated physical properties, compatibility with other materials and ease of synthesis. For example, nanoparticle shape may be controlled by varying the ratios of the host crystal and various solvents, surfactants, monomer concentrations and other constituents involved in the synthesis process. Temperature control is also an important variable in end product quality and performance.
High temperatures are generally necessary for the production of high quality nanocrystals of various chemical compositions. The large selection of high boiling point hydrophobic solvents makes the availability of hydrophobic precursors for the particular kind of nanocrystal, essential. As a classic example, the problem of making a hydrophobic selenide precursor for the high temperature preparation of CdSe nanocrystals has been solved with the use of tri-alkyl phosphine ligands. These readily convert elemental selenium into the selenide anion form and bring it into the hydrophobic reaction solution. As an example, U.S. Pat. No. 7,998,271 B2, issued Aug. 16, 2011, to Alkhawaldeh et al., (“the '271 patent”) describes a synthesis of CdSe nanocrystals that uses trioctylphoshine to render the selenide anion soluble in the hydrophobic reaction environment. However, the '271 patent does not address the above-referenced problems associated with the production of NaYF4 nanocrystals on a commercially viable basis.
A variety of potential new technologies are based upon upconverting nanocrystals, the best performing of which are based upon a NaYF4 host. Synthetic procedures for NaYF4 nanocrystal production have gradually improved over the last decade. The nanocrystals produced by the latest methods exhibit narrow size distributions and high upconversion efficiencies; however, the necessity to use volatile components presents a significant challenge for inexpensive large scale manufacture of these nanocrystals.
Prior to the publication of the synthetic procedure based on NH4F dissolved in methanol and/or water in 2008, precursors such as NaF and CF3COOH have been used as the fluoride source. (Zhengquan Li and Yong Zhang. Nanotechnology, 2008, 19, 345606). More recently, a novel procedure has been reported that utilizes NaYF4 nanocrystals of a less stable, cubic, crystal structure as a precursor for the more stable, hexagonal, NaYF4 nanocrystals. (Noah J. J. Johnson, et.al. J. Am. Chem. Soc. 2012, 134, 11068-11071). These and similar methods have been adapted by the majority of laboratories in academia and have generated the highest quality NaYF4 nanocrystals to date but are not feasible for large-scale commercial production environments.
A notable attempt to remove volatile components from the reaction solution was reported in 2009 by Liu C. et al. published a report in the Journal of Materials Chemistry. Their method allowed for the production of very monodisperse nanocrystals of NaYF4 doped with various rare earth cations. The critical weakness of their approach was the necessity to use excess fluoride precursor to control growth, which stemmed from the low solubility of NaF in the organic solution that lacked a hydrophobic component that could bind fluoride. The extra fluoride ions that are not bound to 3+ cations are free to react with anything in the reaction solution, including the glassware and solvent. An attempt to follow this approach on an industrial scale would carry great risks including, but not limited to fires and release of deadly toxic fumes.
Moreover, the strategy based on organic phosphines that has been successful for CdSe has not been reported in conjunction with and is unlikely to be feasible for the preparation of NaYF4, especially on a large scale. By-products that can potentially form during the course of a high temperature reaction involving fluoride and organic phosphines are closely related to nerve agents such as Sarin. Consequently, development of the analogous hydrophobic precursor for the preparation of NaYF4 nanocrystals requires the identification of new ligands that both solubilize fluoride and do not inhibit the growth of high quality crystals.
Possession of a hydrophobic precursor not only simplifies preparation of a batch of nanocrystals, but also simplifies its modification. Deposition of a “shell” of a given composition onto upconverting NaYF4 nanocrystals is a very popular method for improving their optical properties. Adoption of such a method for large scale manufacture in the absence of a hydrophobic precursor is limited by the necessity to cycle the reaction temperature between two different temperature thresholds (one being less than 100° C. and a second being greater than 300° C.) with every new addition of volatile precursors. Such manipulations become progressively more time consuming and expensive as the scale of the process is increased.
In view of the foregoing, it will be apparent to those skilled in the art that a need exists for an improved and simplified process and associated materials for mass production of high quality upconverting nanocrystals, in particular, nanocrystals based upon a NaYF4 host, without the use of volatile solvents and the formation of undesirable and hazardous toxic side products.